1. Field of the Invention
The present invention relates to a method of forming a semiconductor thin-film and a laser apparatus used therefor. More particularly, the invention relates to a method of forming a semiconductor thin-film applicable to the fabrication of so-called polysilicon Thin-Film Transistors (TFTs), in which desired alignment marks are formed in the thin-film, and a laser apparatus that makes it possible to conduct the method.
2. Description of the Related Art
In recent years, various improved polysilicon TFTs have been developed vigorously as an electronic element for forming an integrated circuit on a glass plate. To form a polysilicon (poly-Si) thin-film (which may be simply referred as “film” hereinafter) used for polysilicon TFTs, the so-called “excimer laser annealing method” has been popularly used. In this method, an amorphous silicon (a-Si) film is formed on or over the surface of a glass plate and thereafter, an excimer laser beam is selectively irradiated to desired parts of the a-Si film for a short period, thereby temporarily melting the a-Si film in the parts due to heat and re-crystallizing the same after cooling in the atmosphere. Thus, the irradiated parts of the a-Si film by the laser beam are selectively turned to poly-Si regions, in other words, poly-Si regions are selectively formed within the a-Si film.
One of the known excimer laser apparatuses applicable to the above-described “excimer laser annealing method”, which has already come onto the market, has a linear aperture of approximately 300 mm×0.4 mm and is designed to generate a laser beam having a linear spot on an object or target. On operation, the beam or spot is scanned along the surface of the target in the widthwise direction of the linear spot at a pitch of several tens micrometers.
However, when the above-described “excimer laser annealing method” is used for forming poly-Si TFTs with the known excimer laser apparatus, there is a known problem that the obtainable characteristics of poly-Si TFTs formed in the irradiated part are likely to be non-uniform. This is because the heating effect of the irradiated laser beam to the a-Si film in the periphery of the irradiated part is different from that in the middle part of the same irradiated part and as a result, the microstructure in the said periphery is different from that in the said middle part. This problem is disclosed in the article entitled “Improving the Uniformity of Poly-Si Films using an Excimer Laser Annealing Method” written by T. Nohda et al., Technical Report of IEICE (SDM92-112), published in December 1992, pp. 53–58.
The above-described problem can be solved by an improved laser annealing method disclosed in the Japanese Patent No. 3163693 issued on Mar. 2, 2001. In this method, TFTs are gathered in the irradiation range of a laser beam, where a uniform energy density (i.e., irradiation intensity) of the laser beam is obtainable. This is to exclude the above-described heating effect difference of a laser beam in the known laser annealing method using a scanning laser beam. The laser beam in the form of pulse is irradiated to the whole range two or more times without scanning.
With the improved method disclosed in the Japanese Patent No. 3163693, the area or size of the irradiation range varies dependent on the energy of the pulsed laser beam. Recently, an improved laser light source has already been developed for this purpose, which generates laser beam pulses with a high energy density in such a way that an area of approximately 40 mm×50 mm corresponding to the size of a LCD (Liquid-Crystal Display) panel for portable telephones is fully irradiated.
In the above-described improved laser annealing method disclosed in the Japanese Patent No. 3163693, it is important to align the irradiation range of a laser apparatus with the TFT region where poly-Si TFTs are formed. To realize this alignment, it is popular to form alignment marks on the glass plate and to provide a video camera for reading or recognizing the marks onto the laser apparatus. This technique is disclosed in, for example, Japanese Non-Examined Patent Publication No. 8-71780 published in Mar. 19, 1996.
However, if a video camera is additionally provided on the laser apparatus, there arises a problem that the structure of the laser apparatus is complicated and at the same time, the dimensions of the apparatus are increased. In particular, the size of a rectangular glass plate for LCDs has now become approximately 1 m×1 m. Therefore, if a mark-reading or mark-recognizing chamber is additionally provided on the known laser apparatus along with the annealing chamber, the footprint (i.e., the occupation area) of the apparatus would be remarkably expanded. Moreover, to align the irradiation range of the laser apparatus with each of the TFT regions of the glass plate, the movable stage of the laser apparatus needs to be not only translational along the X and Y axes but also rotatable in the X-Y plane for θ compensation. At the same time, the said stage needs to be configured in such a way that fine adjustments are possible. As a result, if so, the said stage would be complicated in structure, the fabrication cost of the laser apparatus would be raised, and the rate of operation thereof would be lowered.
Furthermore, if a video camera is provided on the known laser apparatus and alignment marks are formed on the glass plate, a time for reading or recognizing the alignment marks on the glass plate and a time for alignment between the apparatus and the glass plate are essential. Thus, there is a problem that the throughput for the laser annealing process is lowered.
Additionally, to form the alignment marks on the glass plate, a lithography process for forming a mask pattern and an etching process for selectively etching the material for the marks using the mask pattern are necessary. Therefore, there arises a problem that the total number of the necessary process steps for fabricating the TFTs on the glass plate increases.
To solve the above-described problems, an idea that the alignment marks are formed in the laser annealing process is effective. In this idea, an a-Si film is selectively melted temporarily by the irradiation of a laser beam and cooled in the atmosphere, thereby crystallizing the irradiated part of the a-Si film to result in a poly-Si region. Since an a-Si film and a poly-Si film are different in optical constants from each other, alignment marks can be made with a crystallized (i.e., poly-Si) region or regions and a non-crystallized (i.e., a-Si) region or regions.
However, the laser beam diameter for the TFTs is in the order of centimeters (cm) while the size of the alignment marks for a so-called stepper (which is used for a subsequent lithography process) is in the order of micrometers (μm). Therefore, to form the alignment marks with a desired high accuracy, the accuracy of finishing of comparatively large-sized optical elements in the order of 10 cm (which is required for generating a laser beam diameter in the order of cm) needs to be in the order of approximately 10 nm or less (which is required for forming a laser beam diameter in the order of μm). In this case, the fabrication cost of required optical systems for the laser apparatus will be extremely raised compared with ordinary optical systems applicable to forming a laser beam diameter in the order of cm.
Moreover, it is not realistic to provide a mechanism for changing the laser beam diameter from the order of cm to the order of μm for the purpose of forming the alignment marks. Therefore, when the alignment marks are formed, a laser beam diameter in the order of μm needs to be generated with an appropriate mask. In this case, however, there is a problem that a high-resolution mask is necessary. Moreover, there is a possibility that desired alignment marks are not obtained if the height accuracy of the stage, the thickness accuracy of the glass plate, and/or the surface roughness accuracy of the glass plate is/are not so high because of shallowness of the depth of focus of the laser beam.